Table of Contents Introduction Design Considerations



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Table of Contents

Introduction…………………………………………………………..2

Design Considerations……………………………………………….2

Vertical Deflections………………………………………………......5

Raised Crosswalk and Raised Intersection……………………5

Sidewalk Extension……………………………………….. …9

Textured Crosswalk…………………………………………..11

Speed Hump…………………………………………………..11

Horizontal Deflections……………………………………………….13

Chicane………………………………………………………..13

Lateral Shift…………………………………………...16

Curb Extension……………………………………………......16

Center Island Narrowing……………………………...19

Curb Radius Reduction……………………………………….19

Neckdown ……………………………………………21

Choker……………………………………………......21

On-Street Parking…………………………………………….22

Raised Median Island………………………………………...24

Traffic Circle…………………………………………………25

Roundabout………………………………………......28



Obstructions………………………………………………………...29

Directional Closure…………………………………………..29

Diverter……………………………………………………....32

Full closure…………………………………………………..35

Intersection Channelization……………………………….....37

Median through an Intersection……………………………...39

Right In/Right Out Island…………………………………....41

Traffic Calming Signs……………………………………………...43

Signs………………………………………………………....43

Speed Hump sign………………………………………….....43

Traffic Calmed-Neighbourhood sign………………………...44



Appendix A………………………………………………………….45

Glossary……………………………………………………………..46

Bibliography………………………………………………………. .48

Design Guidelines for Traffic Calming Measures
Introduction

The purpose of this guide is to standardize the design procedures of traffic calming measures wherever possible in order to ease the implementation of effective traffic calming strategies in communities throughout North America. The information contained in this guide is not a strict set of standards because of certain instances where specifications may not be able to be met. The specifications presented are optimal, based on various research projects throughout North America, and should be used wherever it is possible and feasible to do so.


Design Considerations

There are many general considerations that need to be addressed when designing any traffic calming measure.



  • Grades: Maximum and minimum grades are put into place in order to minimize vehicle damage, loss of vehicle control, and reduce potential road or property maintenance due to problems such as poor drainage that could arise from traffic calming measures. An example of a minimum grade is raised crosswalks or raised intersections having at least a 1% cross slope for drainage purposes. Maximum grade must be considered when looking at implementing a vertical deflection on a sloped road. For example, the maximum grade for implementing a speed hump is 8%; anything more than this is thought to be potentially hazardous to a driver because of the high transition slope between the speed hump and the road.

  • Long wheelbase and Emergency Vehicles: Knowledge of emergency vehicle routes and primary street usage must be acquired before implementing certain traffic calming measures because of the large turning radii of these vehicles. Many horizontal street calming measures such as traffic circles, chicanes, lateral shifts, and directional closures can be impassable for certain large vehicles, and therefore should be limited to residential use. Another option to accommodate large vehicles is using a mountable rolled curb around the perimeter of the horizontal measure.

  • Ease of Passage for Other Street Users: Any traffic calming measure which sacrifices the safety and general use of the street to cyclists or pedestrians should not be implemented. Compatibility of street calming measures to other street users can often be accomplished fairly easy. For example, when implementing a speed hump, passage for bicycles can be obtained by leaving sufficiently wide gaps in the speed hump near the side of the street. Similarly, any street closure should include a walkway or path so it is still possible for pedestrians to access the neighbouring street.

  • Surface Drainage: Surface Drainage is an important design consideration on both roadways and sidewalks because of the potential problems it can create for both the road and sidewalk condition, and also the safety of their users. All traffic calming fixtures should have a minimum cross-slope grade in order to promote drainage and reduce ponding or ice patches. On roadways with traffic calming measures that do not have drainage gaps, catch basins must be located at a higher elevation than the traffic calming measure if there is any grade to the roadway.

  • Access to Underground Utilities: Whenever implementing a street calming measure, the accessibility of underground utilities must be ensured (Skene, Pg. 4-1 – 4-2).

  • Desired Vehicle Speed: The desired vehicle speed through a traffic calmed area must be determined before the exact design dimensions of the traffic calming device can be specified. For horizontal traffic calming measures, the smaller the turning radius is, the slower the traffic will pass through the area. A formula from mechanics is used to relate the turning radius to the velocity of the vehicle.




R

Maximum horizontal radius of the curve (ft)

V

Velocity of the vehicle around the curve (mph)

e

Superelevation rate

f

Side friction factor

R = V 2

15 (e + f)
The superelevation rate is negligible in almost all urban areas. The side-friction factor is based on safety against skidding and level of discomfort, and can be determined from graphs and tables in the American Association of State Highway and Transportation Officials (AASHTO) “A Policy on Geometric Design of Highways and Streets”. The calculations of the curve radius apply only to single curves. They can only be used as general approximations for horizontal traffic calming measures involving more than one turn such as chicanes and traffic circles.

For vertical traffic calming measures such as speed humps, another mechanics formula can be used to calculate the centrifugal acceleration over a circular hump of a given radius.




A

Acceleration (ft/sec2)

V

Velocity of vehicle over hump (mph)

R

Radius of hump (ft)

Constant

Conversion Factor

A = 2.15 V 2



R
International traffic calming standards suggest that a vertical acceleration of up to 1 g (9.81 m/sec2) can be tolerable for short periods, however this is fairly high so personal discretion may be used to determine an appropriate sized speed hump (Ewing, pg. 67 – 69).

  • Maintenance: Increased or more difficult street maintenance may occur as a result of traffic calming. Certain measures should be considered to minimize this negative effect. Snow plowing over vertical traffic calming elements may cause damage either to the snow plow or the street. Other traffic calming options or careful snow plow operating will be necessary to prevent damage on frequently plowed streets. Accumulation of snow or leaves may also occur as a result of some traffic calming measures so frequent removal may be required at certain times of the year.

  • Materials: Materials for all traffic calming measures that will be used in sub-zero temperatures should be able to withstand repeated freeze-thaw action and heavy loads at least as well as the surrounding roadway material. Care should be taken to avoid any materials or paints that may be excessively slippery when wet.

  • Signage: Though signs are usually required to inform road traffic of the traffic calming measure approaching, additional warning signs should be minimized for aesthetic reasons. It is unnecessary to erect any additional signs; if the traffic calming measures are navigated at the posted speed there is no risk of vehicle damage or loss of control (Skene, Pg. 4-2)

  • Temporary Installations: Temporary traffic calming installations are often beneficial to determine an optimal location or specific design of a traffic calming measure before investing in a permanent fixture. Another advantage of a temporary installation is to judge community response to the device. Disadvantages of temporary traffic calming measures are primarily based on poor aesthetics, creating a public disapproval even for a similar permanent fixture that may be more pleasing. If a temporary traffic calming measure is aesthetically pleasing, it may be able to become a permanent fixture (Ewing, pg.82).

  • Streetscaping/Landscaping: Adding streetscaping or landscaping to a traffic calming measure may considerably increase the cost and maintenance but can improve the measure both aesthetically and functionally. Effective landscaping on street calming measures can improve community acceptance by softening the appearance and giving a higher perceived functionality. Streetscaping and landscaping can also make a road seem narrower therefore reducing speeds. Specific streetscaping elements can include ornamental items such as lighting, benches, and planters as well as vegetation. When incorporating streetscaping or landscaping elements, it is important to maintain good visibility and not sacrifice street user safety (Skene, 4-2)

To maintain the highest level of safety and appearance, it is essential to adequately maintain all landscaping measures. There are a few different options when it comes to maintaining streetscaping and landscaping elements. The city of Seattle is a good example because of its extensive use of street calming measures. Seattle initially did all of the landscaping and maintenance for their traffic circles. As more and more circles were constructed the cost of maintenance became unaffordable for the city, so a community involvement program was put into place. The city does the initial landscaping of the measure and the neighbourhood residents are then responsible for any maintenance or replacement that may be necessary. This method generally worked well for Seattle, but there were some problems with neighbourhood negligence resulting in a wide range of landscaping quality on the traffic circles. Other cities such as Portland take full responsibility for construction and maintenance of their streetscaping and landscaping measures for liability and quality purposes (Ewing pg.82).

Vertical Deflections

Vertical deflections include raised crosswalks, raised intersections, sidewalk extensions, speed humps and tables, and textured crosswalks. Vertical deflections are most effective in reducing traffic speeds rather than traffic volumes.


Raised Crosswalks and Raised Intersections - raised junctions, intersection humps, plateaus

Raised Crosswalk serving a dual purpose as a speed hump (Delaware Register of Regulations, Pg. 542).


A raised crosswalk can be implemented either mid-street or at an intersection as long as the boulevard is wide enough to accommodate the required elevation change at the maximum grade. The height of the raised crosswalk depends largely on desired vehicle speed (see Design Considerations), but should generally be high enough to discourage speeding but not produce a risk of vehicle damage or loss of control when navigated at the intended speed. Speed tables can also serve a dual purpose as raised crosswalks.

Raised Intersections are flat raised areas covering an entire intersection with ramps up on all sides (Ewing, pg.34). The flat top is often finished in brick or other suitable textured material for increased effectiveness and increased aesthetics. Like raised crosswalks, the height of the intersection depends on desired traffic speed, but should generally be suitable to reduce speeding while eliminating risk to drivers and their vehicles. Emergency and transit vehicle volume in the area should also be considered due to the decreased ease of use of large quickly moving vehicles on raised intersections, especially when turning.



Design Dimensions

The height of raised intersections and raised crosswalks should correspond throughout the community’s street network in order for citizens to get a better grasp of the design speeds and operations of the measure. A typical height for an intersection or crosswalk with a standard 2m ramp would be 80mm and could change according to the length of the ramp. The location of a raised crosswalk or intersection in relation to sidewalks and curbs should be identical to regular crosswalks or intersections. A minimum 15mm lip should be maintained at all sidewalk-crosswalk transitions to aid the visually impaired in acknowledging the presence of the intersection or crossing. The transition area between the sidewalks and the raised intersection should be lowered to a 15mm curb face height with a transition slope of no greater than 6%. This transition area should also have a textured finished such as stamped concrete to provide a physical indication of the presence of the crossing. Recommendations for these textured surfaces can be found in the “Geometric Design Guide for Canadian Roads” (GDGCR). The flat top of the raised intersection should have a minimum grade of 1% to promote drainage. The dimensions shown have proven to be the best compromise for emergency and transit vehicle passage, effective speed reduction, and drivability. Raised crosswalks and intersections should be keyed directly into the existing pavement to provide the most secure connection



Signing Requirements

A speed hump sign should be located directly beside a raised crosswalk facing traffic. If a raised crosswalk is located on a one way street, a speed hump sign should be placed on either side of the street facing traffic. The speed hump sign is necessary for a raised intersection in the same manner, unless the intersection is stop sign or stop light controlled in which case no sign is needed. Pedestrian crosswalk signs are necessary on either side of the raised crosswalk. Solid white reflective pavement markings such as arrows should also be included directly preceding raised crosswalks and intersections. No advance warning signs are recommended for either of these traffic calming measures in order to reduce redundancy and improve aesthetics.



(Skene, Pg. 4-4)



(Skene, Pg. 4-5)





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